Combination metal and composite fasteners exhibit a relatively high strength-to-weight ratio that renders them usable for aircraft application, particularly fastening of skin panels. However, since an aircraft is exposed to lightening strikes in flight, precaution must be taken to dissipate the electrical energy of the lightening strike without compromising the structural integrity of the airframe. This has heretofore presented a problem since composite materials are generally less conductive than the metallic components of the fasterner. When electrical continuity across the exterior surface of a composite airfoil is disrupted, an electrical potential tends to build up, often resulting in an arc which, in some cases, can destroy the fastener.
A specific problem that is addressed by the instant invention is the problem experienced when an unshielded electrically conductive bolt, rivet, or the like, is seated in an aperture in a composite structure. When lightening strikes the surface of the composite structure, the electrical energy is conducted by the carbon filaments thereof until the metallic portion of the bolt or fastener is reached. Because the bolt presents less electrical resistance than the carbon fibers of the composite skin, the electrical energy tends to follow the bolt. However, because the conductive path offered by the bolt is discontinuous, the structural integrity thereof is often compromised due to subsequent capacitive discharge of the electrical energy.